1. Field of the Invention
The present invention relates to the field of electronic amplifiers. More particularly the present invention relates to the use of vacuum tube circuits to provide amplification of electronic signals.
2. Description of The Prior Art
Vacuum tubes (hereinafter called "tubes") are considered obsolete except in some specialized fields such as radio frequency transmission where power amplifier tubes have not been completely replaced by transistors. Transistors have almost completely replaced tubes for small signal amplification and signal processing. There still exists a public interest in tube-based audio equipment, however.
Tube based home entertainment audio systems are increasingly in demand. There exists a growing group of audio enthusiasts who believe tube audio equipment sounds more lifelike or musical than equivalent transistor equipment. Tube based professional audio products have recently come into greater demand in the wake of the digitalization of the audio media because of an increasing awareness of digital audio's unnatural distortion. Analog tube devices such as compressor/limiters and equalizers are being used to recreate nuances of sound lost to digital quantization and processing or to soften the "edge" of digital sound. The demand for tube based equipment now supports a growing list of manufacturers in what might be called a resurgence of tube technology in audio. It would therefore be desirable and useful to create a new and improved tube amplifier circuit.
Field effect transistors (FETs) have been tried as a substitute for tubes to improve the reliability and stability of amplifiers, with limited success. While FETS are similar to tubes in many ways, they do not duplicate the sound reproduction of a tube circuit. It would therefore be useful and desirable to combine the sophistication and reliability of transistor technology with true tube technology in such a way so as to create products which deliver the sound quality of tubes but have much greater reliability and stability.
A problem of conventional tube circuits is that they require a high supply voltage of several hundred volts. This makes it inconvenient to integrate tubes into a solid state circuit design.
It would be an improvement over prior art to find a way to efficiently use tubes at the much lower power supply voltages generally used by transistors, for example only and without limiting the present invention, 10 to 30 volts. It would also be an improvement over prior art to find a way to operate tubes at a relatively low level of plate power dissipation while still obtaining useful transconductance.
It would further be an improvement over prior art to devise a method to operate tubes in such a way that the input control grid retains its normal characteristics and influence upon the plate current, but to develop the output signal voltage at a relatively low output impedance. It would also be desirable, if possible, to reduce or eliminate the tube's miller capacitance.